1. Technical Field
The present disclosure relates to matching devices for matching circuit impedances using variable-capacitance diodes, and antenna matching circuits using the matching devices. In particular, described is a technique for shifting a variation range of a voltage applied to a variable-capacitance diode to increase a variable capacitance range of the diode.
2. Background Art
In radio communication in general, to increase reception efficiency, impedance matching between an antenna and a receiving circuit is provided (impedance matching) in accordance with the reception frequency of a radio signal. In a scheme for performing such impedance matching, a matching device including a variable-capacitance diode is disposed between an antenna and a receiving circuit.
In addition, a variable-capacitance diode is also used in a tuning circuit for extracting only a reception frequency component from radio signals received by an antenna which contain a plurality of frequency components, so that a tuning frequency (resonant frequency) viewed from a receiving circuit is changed.
Variable-capacitance diodes are also called varicap diodes or varactor diodes, and generate capacitances in accordance with the widths of depletion layers produced therein. When applied with a control voltage which serves as a reverse bias voltage, the width of the depletion layer of a variable-capacitance diode changes, which as a result makes the capacitance variable.
For example, Japanese Unexamined Patent Application Publication No. H10-336055 (Patent Document 1) discloses an antenna tuning controller having an antenna including inductance elements and a variable tuning section for setting the resonant frequencies of the inductance elements in accordance with a control voltage, and a control means for supplying the control voltage to the antenna. In this variable tuning section, a serially connected fixed-capacitance element and variable-capacitance element are connected in parallel with the inductance elements.
In addition, Japanese Unexamined Patent Application Publication No. H11-017580 (Patent Document 2) discloses an antenna matching device provided with an antenna matching section having a variable-capacitance element, an operation section for performing adjustment processing in accordance with the frequency characteristic of the antenna matching section and adding a constant output from a constant generating section, and a D-A converter for performing analog conversion of output data from the operation section so as to output a control voltage for the variable-capacitance element.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-336055
Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-017580
As described above, in a variable-capacitance diode, the amount of capacitance, which varies in accordance with an applied reverse bias voltage, i.e., the variable capacitance range, depends on the variation range of a control voltage applied across the variable-capacitance diode.
On the other hand, in particular, in a mobile receiver (communication device) such as a compact radio set or a mobile phone, there has been a trend toward a low voltage of driving power (for example, as low as 1.8 V) used in a matching circuit, along with the recent trends toward high density and low power consumption of a semiconductor integrated circuit used in circuits in the mobile receiver. Along with these trends, the variation range of a control voltage which can be applied across a variable-capacitance diode is also decreased, which further limits the variable capacitance range.
The driving power may be boosted using a charge pump and a DC-DC converter. However, this may complicate the circuit configuration and increase the weight, and thus it is not realistically possible to employ such a scheme.
For the above reasons, there is a problem in that the variable capacitance range of a variable-capacitance diode is limited and the performance as a matching device is degraded.